Project Summary Epilepsy is a serious neurological disorder characterized by spontaneous seizures and increased risk of mortality, especially for one-third of the individuals with chronic seizures that cannot be controlled in spite of optimal drug therapy. Currently, the most effective treatment for drug-resistant seizures is resective surgery and in cases such as temporal lobe epilepsy (TLE) with hippocampal sclerosis, the most common form of drug- resistant epilepsy, surgery can reduce or eliminate seizures in most patients. However, it does not help all patients, particularly those with seizures suspected to begin in the temporal lobe but without a MRI lesion or MRI contains a lesion in hippocampus and neocortex or scalp EEG indicates bilaterally. In situations like these the extent of the epileptogenic zone (EZ), which is the brain area that is necessary and sufficient for generating seizures and minimum resection necessary for seizure relief, is uncertain. There is tremendous interest in studies of the epileptogenic network (EN), i.e. the larger brain region that supports the generation and spread of seizures and manifestation of epilepsy, and we hypothesize in difficult cases of TLE, knowledge of the EN could help us to localize the EZ. Even with recent progress from studies of the EN, we still do not know the full extent of brain abnormalities that define the EN, what part is necessary for generating seizures, or how much of the EN needs to be removed to eliminate seizures. This project addresses these issues and we have developed a comprehensive approach to study them, including novel structural and diffusion MRI and EEG functional connectivity tests, as well as studies of high frequency oscillations (HFOs), which are a potential biomarker of the EZ. We will apply this approach to presurgical patients with suspected TLE who require invasive EEG tests as part of their standard diagnostic care. To help us define the EN in Aim 1, we will evaluate structural MRI for gray matter abnormalities and morphological covariance between brain areas; in Aim 2, we will assess diffusion MRI for white matter microstructural and fiber tract anomalies; in Aim 3, we will use a new approach called gamma event coupling in combination with unit and HFO recordings to investigate the functional connectivity encompassing the EN and identify which parts of it are generating seizures and spread; and lastly in Aim 4, we will determine which components of the EN and how much of it needs to be removed to eliminate seizures. In this project, defining the EN should provide important information on the structural and functional mechanisms generating seizures, and ultimately this will help us to localize the EZ, improve diagnosis, advance surgical and non-surgical therapies, develop new therapies, and increase the likelihood for seizure freedom.